The present invention relates generally to radio frequency communication systems and in particular to switching devices for switching between various frequency bands. The present invention is a device and method for switching two radio frequency signals that have relatively close or even the same frequency band.
Cellular phone systems are rapidly expanding throughout the world. As the requirement for cellular communication increases the number frequency bands used to communicate will also increase. Additionally, as different standards are implemented, they may also require different and narrower frequency bands on which to conduct the communications. Close frequency bands such as 1750 MHz /1900 MHz /2100 MHz may be implemented on a variety of systems. For example operating frequencies for some existing systems include PCN (Personal Communication Network) 1710–1800 MHz, PCS (Personal Communication System) 1850–1990 MHz, and DECT (Digital European Cordless Telephone) 1880–1900 MHz. Additional systems such as WCDMA (Wideband Code Division Multiple Access), GSM (Global System for Mobile Communication), DCS (Digital Cellular System), and the like, may require additional bands in a limited range of frequencies. Additionally each system can require different circuits/components (Tx/Rx switches, amplifiers, etc.) in the signal paths to be combined at a common terminal, for instance an antenna. It is desirable to be able to access multiple systems as not all geographic areas are served by the same system. Therefore, it is desirable to be able to support various systems with one device. To do so, it is preferred to connect various signal paths to a common terminal to avoid the substantial costs and space requirements of providing separate devices for each system used.
Some communication systems allow for multifrequency systems that have frequency bands that are widely separated. For instance, current GSM systems operate at 900 MHz whereas DCS systems operate at 1750 MHz. The wide frequency band separation allows for a frequency diplexer system as shown in FIG. 1 and described in published PCT application WO 99/52172 and related copending U.S. application Ser. No. 09/287,546, the content of which is hereby incorporated by reference in its entirety. The system has a low band input 100 and high band input 200 coupled at a common output 300 such as an antenna. The low frequency signal path includes a filter formed by inductor L101, and capacitors C101 and C102. Similarly, the high frequency path additionally includes a filter formed by inductors L201 and L202 and capacitor C201. The filters each have a passband corresponding to the frequencies of the respective low band and high band frequencies and correspondingly block the other frequencies. The wide separation of the low and high frequency bands allows the filters to conduct the desired low band 100 or high band 200 frequency signal and reject the non-desired frequency signal. For example, when the low band input 100 is active, the filter 205 in the high band path effectively blocks the low band output coupled at the output 300 from reaching the high band input 200. Similarly, when the high band input 200 is active, the filter 105 in the low band path effectively blocks the high band output coupled at the output 300 from reaching the low band input 100.
Although the diplexer system depicted in FIG. 1 can be useful in applications where the frequency band separation is wide, it would not be effective for systems where the frequency bands are relatively close such as DCS at 1750 MHz and GSM-1900 at 1900 MHz, which is a new alternative frequency band for the GSM system. Specifically, when the frequency bands are relatively close, the filters could not provide both a passband to allow a signal to conduct from the input to the common output at the desired frequency, and still block the non-desired frequency when the other input is conducting.